Cysteine-protected lignin as a fully bio-based adhesive for high-performance wood bonding

L-cysteine-protected lignin (CPL) is isolated from biomass in aqueous formic acid. • CPL adhesive achieves a record wet bonding strength of 1.48 MPa. • Multi-layer plywoods bonded with this adhesive exhibit practical viability. • Physicochemical crosslinking enables robust adhesion at the bonding interface. To alleviate the dependence of commercial adhesives (e.g., phenol-formaldehyde and urea-formaldehyde resins) on fossil resources, substantial research efforts have focused on developing lignin-phenol-formaldehyde (LPF) resins. Owing to the low reactivity of condensed technical lignins (e.g., kraft and organosolv lignins), LPF resins continue to suffer from structural heterogeneity, inferior bonding performance, limited phenol substitution rate, and formaldehyde emissions. Given that lignin functions as a natural binder to endow higher plants with structural rigidity and water resistance, extracting native-like lignin from lignocellulosic biomass therefore offers a promising route towards fully bio-based alternatives. However, simultaneously achieving green lignin extraction and practical bonding performance of the resulting adhesives remains elusive. Here we report an effective strategy to overcome this trade-off. Our approach involves protecting lignin with L-cysteine in an aqueous formic acid system during biomass fractionation, followed by simply dispersing the resulting L-cysteine-protected lignin (CPL) in water. Among fossil-free all-lignin adhesives, the resulting CPL adhesive achieves a record wet bonding strength of 1.48 MPa and an exceptional dry bonding strength of 1.85 MPa. Multi-layer plywoods that bonded with this adhesive also exhibit superior flexural resistance, long-term stability, and negligible formaldehyde emissions. Mechanistically, L-cysteine forms C–S bonds at the α-positions of lignin during fractionation, which controls the initial molecular weight of CPL via inhibiting condensation reactions, and preserves its active linkages (e.g., aryl ether bonds) and functional groups (e.g., hydroxyl and carboxyl groups). These structural features facilitate the penetration, in situ depolymerization and condensation of lignin, as well as non-covalent interactions during hot pressing, thereby enabling the high-performance adhesion of CPL adhesive towards wood veneers. Download: Download high-res image (153KB) Download: Download full-size image